The present invention relates to data communications equipment including, for example, voiceband modems and data service units.
Historically, equipment used to communicate digital information over, for example, the voiceband telephone network performed only the most basic of functions--principally the modulation of outgoing data onto outbound line signals and demodulation of incoming line signals to recover incoming data therefrom. Indeed, it was in these early days that the term "modem",(MOdulator/DEModulator), was coined. As the art evolved, the capabilities of modems--as well as their digital transmission counterparts, referred to as "data service units" or DSUs--were expanded to provide more sophisticated functionalities, such as adaptive equalization, multidimensional modulation and trellis coding. The continuing evolution of this so-called "data communications equipment" (DCE) enabled modem designers to provide devices capable of operating at increasingly higher bit transmission rates. In addition, the advent of microprocessor technology enabled modem designers to build greater functionality into their products, notably sophisticated modem and network diagnostic capabilities.
Notwithstanding the continued advances in DCE functionality, as just described, there has been little advance in the techniques used to interconnect the DCE with its co-located data terminal equipment (DTE), e.g., mainframes, data terminals, personal computers, etc., and with various other adjunct, or application, co-located devices, such as encryptors, compressors and multiplexers. For example, one approach that is still in widespread use is the use of hard-wired, point-to-point connections. Disadvantageously, however, this results in configurations that are both difficult and expensive to administer, at least in installations having a significant amount of equipment. For example, tracing cables under the floor or through crowded cable races in order to make desired interconnection changes or to trouble-shoot is both time-consuming and labor intensive. Significant down time may also be encountered unless personnel are deployed on site on a round-the-clock basis. Use of patch panels, representing a somewhat more sophisticated approach, solves the cable-tracing problem but, again, is personnel-intensive.
The above drawbacks could be substantially overcome using a stand-alone matrix switch controllable from, say, a central control site. Matrix switches are, however, bulky and quite expensive. Modern day time-division-multiplexed-bus-based digital PBXs, many of which are capable of switching not only voice, but also data signals, have been purchased by users and employed exclusively in a data mode as a way of interconnecting DTE with DCE. However, even though digital PBX technology has been available for quite a few years, vendors of DCE--notably voiceband modem vendors--have not by and large incorporated PBX- or PBX-like-interconnection architectures in their product lines.